The components of electronic cameras require low power consumption, low weight and cost efficiency. These design criteria are challenged by the demand for optically adjustable cameras that provide autofocus, zoom optics, or both. These features require the relative movement of optical elements to provide the adjustment. The required motion is typically linear but may use a rotating motor combined with a motion-converting mechanism such as a lead-screw. The motion range is often in the order of millimeters.
When a camera has a movable lens or lens group for focusing or zooming, the exact position of the lens(es) needs to be determined in order to adjust correctly the actuator(s) moving the lens(es). This is required, for example, to accomplish automatic focusing. The problems in such a position measurement are related to the required very high accuracy and linearity of the measurement. Requirements for mechanical strength and reliability are also high because of the amount of duty cycles (>100,000) over the lifetime of the product. Generally, when speaking of optics, the accuracy requirement is very high. The tolerance is usually a few microns. In addition to being accurate, the position determination has to be rapid as well. The objective is to correct a defocused image before the user even recognizes it. This means that the lens position determination and the following corrective lens movement has to take typically place in a few hundredth part of a second. Additionally, current consumption always needs to be minimized. Ideally, the position measurement sensor would also be small and compact in size, as well as economical to manufacture.
U.S. Pat. No. 6,710,950 and U.S. patent application Ser. No. 10/315,885, both assigned to Nokia Corporation and incorporated herein by reference, are both directed to digital camera systems that incorporate the use of adjustable camera optics. Zoom modules, such as the module shown schematically in FIG. 1, are developing to be increasingly compact. The module shown in FIG. 1, shown generally at 100, includes a support tube 102 and first and second lens tubes 104 and 106. In a zoom module, such as that shown in FIG. 1, there are a pair of lens groups that are arranged to move with respect to each other and with respect to the support tube 102. In FIG. 1, the focusing lens (or lens group) is arranged in the left end of the first lens tube 104. The zoom lens (or lens group) is arranged inside the second lens tube 106. The first and second lens tubes 104, 106 have both their own actuators (not shown). In FIG. 1, for example, the second lens tube 106 is arranged to be actuated via a cantilever 108, which protrudes out through an opening 110 provided in the support tube 102, and connects further to an actuator outside the support tube 102. In contrast, in an autofocus system, only a single actuator and a lens tube or corresponding structure may be required. It should be noted that FIG. 1 shows only one possible arrangement for miniature zoom optics. Other distance measurement techniques are based upon resistance, capacitance and inductance.
The motors/actuators for moving and adjusting the lens tubes 104 and 106 require accurate positioning systems, which face the same strict space limitation problems. These space limitations are particularly noteworthy in digital cameras, and particularly digital cameras incorporated into devices such as mobile telephones. In conventional cameras, the lenses have traditionally been moved through the use of electromagnetic stepper motors, and the position information for lenses is received by counting the number of drive signals that are fed to the motor coils. However, such systems, particularly systems including stepper motors do not have a sufficient level of accuracy for use in digital cameras.
In FIG. 1, separate position sensors for the first lens tube 104 and the second lens tube 106 are depicted schematically with reference numerals 112 and 114, respectively. From the above, it is clear that there is a real need for small sized and high precision position sensors in these type of applications. Price is also a very critical point for a position sensor in mass produced products, and at the same time the system should meet the high accuracy requirement that camera optics set. This is particularly important as such cameras are incorporated into smaller and smaller devices, such as portable electronic telephones.
Prior art lens position sensors based on magneto-resistive sensors are disclosed in U.S. Pat. Nos. 5,859,733 and 5,587,846. Additionally, Hall-effect elements are known to be used for similar purpose. One such device is discussed in U.S. Pat. No. 5,587,846. U.S. Pat. No. 5,391,866 discloses an optical lens position sensor which is based on the use of a photo emitter arranged behind a slit and arranged to further illuminate a position sensitive photodetector. However, there is still clear need for small sized and economical position sensors having high accuracy. In particular, non-contacting type optical sensors have not been widely and effectively applied to these kinds of applications.